The present invention relates to chemical milling for removal of a portion of the metal surface of a monolithic metal product.
Chemical milling is often employed to remove a thin skin or layer of the parent metal from the outermost surface of a monolithic metal product so as to provide a desired surface finish or thickness of the metal product. To that end, as used herein, “monolithic metal product” is a reference to a metal member that has a generally homogeneous metal composition, at least through a substantial depth from the surface, where a portion or skin layer of the metal composition is to be removed while leaving behind a surface layer of essentially that same metal composition. In that context, chemical milling refers to a process for the selective and controlled removal of a depth of metal from the surface of a monolithic metal product through the use of chemical etchants so as to expose as a new “surface” a portion of the monolithic metal product that was below, and otherwise integrally part of, the original, and now removed layer or skin. Typically, the thickness or skin layer of metal removed during such a chemical milling process is on the order of 1-50 mils, wherein the depth of the monolithic metal member from which the skin layer is to be removed is typically substantially thicker, such as on the order of several times thicker to as much as several orders of magnitude thicker. Chemical milling may be thus used for most any type of metal for such purposes as achieving certain tolerances, removal of sub-surface impurities in the metal, and elimination of surface microcracks.
Where chemical milling is employed to improve the surface finish as above described, it has been the typical practice to employ highly concentrated baths containing acid or salt mixtures at very high temperatures. It would be desirable to utilize lower concentration baths and at reduced temperatures for the chemical etchant used in chemical milling processes without adversely affecting metal removal rates. In my aforementioned patent application, I teach that an oxide layer, known as a mill scale layer, may be removed from the surface of a monolithic metal product such as a steel sheet or steel bar, for example, with a low temperature electrolyte bath of a dilute solution of acid or base or mineral salts in water. In the process disclosed in my aforementioned patent application, referred to as a pickling process, the metal product is dc coupled to a separate counter electrode having a higher potential (E°) than that of the metal product from which the mill scale layer is to be stripped, and the metal and counter electrode are immersed in or otherwise associated with an electrolyte bath of relatively weak (diluted) acids or bases, at relatively lower temperatures and without imposition of an external positive voltage from the electrode to the metal structure. That process causes removal of the mill scale, at least in part, because a skin layer or portion of the surface of the monolithic metal product is itself removed in the process thereby freeing the mill scale from the surface. Hence, the pickling process there-described is an example of chemical milling.
The present invention provides chemical milling of a monolithic metal product not only for removal of mill scale in accordance with my prior application, but for providing a desired surface finish to or thickness of the monolithic metal product previously provided by chemical milling processes but without the use of the highly concentrated and/or high temperature acids previously used. Rather, as described in my prior application, a monolithic metal product and a separate counter electrode having an E° higher than that of the metal product are exposed to a low temperature bath of diluted acid or base solution and dc coupled without imposition of an external positive voltage for a time to cause a skin layer of the surface to be chemically milled therefrom to provide a desired surface finish or thickness of the metal product. Where there is also an impurity or undesired layer atop the monolithic metal product surface, that impurity is also washed away with the skin layer removed by the chemical milling. Thus, mill scale is removed, for example, from iron and steel products.
In accordance with a further feature of the present invention, non-metallic impurities or deposits may also be removed from the monolithic metal product surface by chemical milling with the above process. More particularly, and by way of example, it is a common problem in boiler tube systems, such as used in steam and power generating plants, that a scale or deposit of non-metallic compounds, such as carbonates, sulfonates, or other chemicals, will build up inside the boiler tubes from the high temperature water passing therethrough. The flow rate of the water begins to slow and can even become blocked off in some of the tubes. In order to repair such boiler tubes, it has been the practice to actually cut out complete sections of boiler-tube wall containing the clogged tubes, and replace the section with a new section of new, fully clear tubes. That process is costly, painstakingly slow, and fraught with dangers and problems for those who perform such work. With the chemical milling of the present invention, a dilute, low temperature acid or base electrolyte may be introduced into the partially clogged tubes, with the tube walls dc coupled to a higher E° counter electrode which is also exposed to the electrolyte. The result is to chemically mill a skin layer of the interior surface of the tubes, thereby also freeing the scale or deposit adhered to the skin layer. The skin layer and impurities may then be flushed out of the tubes along with the electrolyte. Simultaneously, dilute acid may be used to dissolve an element such as a carbonate.
One prior mill scale removal method discussed in Sumita et al. U.S. Pat. No. 4,588,488 proposes to reduce the temperatures and concentrations used in the pickling process for steel with an electron injection method based on cathodic polarization. In this method, a platinum electrode and steel part are immersed in a wash liquid containing an electrolyte, a pH regulating agent and a complexing agent. The positive cathode of an external dc power source is coupled to the electrode, and the negative anode is coupled to the steel. By imposing a positive voltage across the electrode to the steel, the oxide layer is said to be brought into an unstable region by shifting the potentials of the oxides in the base direction from the natural potential to the cathodic polarization potential. At this potential, the oxide is said to be unstable and will dissolve while the metal iron is stable and protected from corrosion. By this method, it is said to be possible to reduce the temperature and acid concentration of the wash bath while still achieving acceptable stripping times.
The method proposed in the Sumita et al. patent is believed to have many deficiencies, and is not believed readily applicable to chemical milling generally. On the one hand, the potential that is applied must be regulated and adjusted in accordance with the actual potentials encountered during the process. The nature of the oils that build up in a pickling tank, and the behavior of the materials therein, make it difficult, if not practically impossible, to monitor the various components and properly control the applied potential. Moreover, the electron injection method is not believed to produce any meaningful improvement in pickling process throughput.
With the chemical milling process of the invention, a skin layer of a monolithic metal product is removed without high concentration and/or high temperature acids, but in a short amount of time and without the need for an external power source, and particularly without imposition of an external positive voltage as proposed by Sumita et al. One example of the present invention is removal of mill scale as in my prior application. Other examples involve surface finishing by removing a skin layer from a monolithic metal product to provide the desired finish or thickness of the product, or to otherwise remove impurities from the surface of the monolithic metal product by stripping off the underlying skin of the product's surface. Additionally, the effect of the natural E° differential may be expanded by connecting an external voltage in the negative sense from the counter electrode to the metal product as opposed to the positive sense of Sumita et al. The negative potential need not be carefully regulated or controlled in relation to the bath or materials, and so is easier to apply and utilize than the positive potential of Sumita et al. and yet is believed to increase the rate of removal of the skin layer.
In accordance with yet a further feature of the present invention, the electrolyte bath may be acidic or basic in nature. In other words, the bath has a substantially non-neutral pH. The pH of an acid bath is advantageously less than 4, more advantageously less than 3, and most advantageously between −1 and +2. The pH of an alkaline bath is advantageously greater than 8 or 9, and more advantageously greater than 10.3.
In accordance with yet a further feature of the present invention, the electrolyte bath may be agitated, and this agitation may be ultrasonic. In the past, ultrasonic agitation has been achieved by inducing the agitation through the wall of the tank, and has been limited to metal tanks by virtue of the metal being better at conduction of the frequency than any other material. Ultrasonic agitation could not be used, however, for brick-lined tanks. Moreover, the metal tanks were subject to corrosion as a result of the metal being subjected to the high frequencies, which cause stress fractures in the material. In the present invention, a transducer encased in metal, such as HASTELLOY® C-276, is inserted into the electrolyte bath. The wave from the ultrasonic transducer hits the part to be chemically milled and bounces back thereby acting like a shock wave, first compressing, then expanding to cause the agitation. Because the agitation is induced through the electrolyte, this method of agitation may be used for any tank material.
By virtue of the foregoing, there is thus provided a chemical milling process that avoids the need for highly concentrated, high temperature acids, but which is easy to deploy and control, and may do so at a faster rate than prior chemical milling processes. These and other objects and advantages of the present invention shall become more apparent from the accompanying drawings and description thereof.